It is well known to protect a bank of a watercourse or natural or artificial lakes by means of a layer of interlocking concrete blocks, preferably overlying a filter mat which further protects the material of the bank from erosion. The present applicant has had extensive experience in the use of such blocks, which are commonly formed with complementary pairs of spigots and sockets, the spigots on the one sides of a course of blocks engaging the sockets on the other sides of the blocks in the next course. If the courses are lapped in the usual manner, the resulting bed of blocks is interlocked in three dimensions, in the manner shown in FIG. 8 of Canadian Pat. No. 957,169. In order to facilitate laying of the blocks, it has also been proposed to use blocks which are tongued and grooved as shown in German Offenlegungschift No. 25.11006. This arrangement provides interlocking in only two dimensions although the extended engagement between adjacent courses tends to compensate for this to some extent. In practice, beds of blocks of these types can prove somewhat deficient in their resistance to wave action such as may occur at the shores of large lakes.